1. Field of the Invention (Technical Field)
The present invention relates to cyclic peptides with a C-terminus free acid, use thereof in pharmaceutical compositions for the treatment of sexual dysfunction in animals, including both male erectile dysfunction and female sexual dysfunction, and methods for the use and administration of such cyclic peptides and pharmaceutical compositions.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Sexual dysfunction, including both penile erectile dysfunction or impotence and female sexual dysfunction, is a common medical problem. Significant effort has been devoted over the last twenty or more years to develop methods, devices and compounds for treatment of sexual dysfunction. While more effort has been undertaken for treatment of penile erectile dysfunction, female sexual dysfunction is also an area to which significant research and effort has been devoted.
At present, one commonly used orally administered drug for treatment of sexual dysfunction in the male is Viagra®, a brand of sildenafil, which is a phosphodiesterase 5 (PDE-5) inhibitor, increasing the persistence of cyclic guanosine monophosphate and thereby enhancing erectile response. Another drug approved in Europe for treating male erectile dysfunction is Ixense®, a brand of apomorphin that is a non-selective dopa receptor agonist. Oral and nasal formulations of apomorphin are currently undergoing clinical evaluations in the United States. There are several other medical treatment alternatives currently available depending on the nature and cause of the impotence problem. Some men have abnormally low levels of the male hormone testosterone, and treatment with testosterone injections or pills may be beneficial. However, comparatively few impotent men have low testosterone levels. For many forms of erectile dysfunction, treatment may be undertaken with drugs injected directly into the penis, including drugs such as papaverin, prostaglandin E1, phenoxybenzamine or phentolamine. These all work primarily by dilating the arterial blood vessels and decreasing the venous drainage. Urethral inserts, such as with suppositories containing prostaglandin, may also be employed. In addition, a variety of mechanical aids are employed, including constriction devices and penile implants.
A number of other agents have been shown to induce or facilitate penile erection in laboratory animals. These include very diverse classes of ligands such as oxytocin (Benelli A, Poggioli R, Luppi P, Ruini L, Bertolini A, Arletti R, Oxytocin enhances, and oxytocin antagonism decreases, sexual receptivity in intact female rats. Neuropeptides 27:245–50 (1994)), vasopressin, vasoactive intestinal peptide, melanotropins, and ACTH as well as their analogs.
A variety of treatments have also been explored for female sexual dysfunction, including use of sildenafil, although the Food and Drug Administration has not specifically approved such use. Testosterone propionate has also been employed to increase or augment female libido.
Melanocortin receptor-specific compounds have been explored for use of treatment of sexual dysfunction. In one report, a cyclic α-melanocyte-stimulating hormone (“α-MSH”) analog, called Melanotan-II, was evaluated for erectogenic properties for treatment of men with psychogenic erectile dysfunction. Wessells H. et al., J Urology 160:389–393 (1998); see also U.S. Pat. No. 5,576,290, issued Nov. 19, 1996 to M. E. Hadley, entitled Compositions and Methods for the Diagnosis and Treatment of Psychogenic Erectile Dysfunction and U.S. Pat. No. 6,051,555, issued Apr. 18, 2000, also to M. E. Hadley, entitled Stimulating Sexual Response in Females. The peptides used in U.S. Pat. Nos. 5,576,290 and 6,051,555 are also described in U.S. Pat. No. 5,674,839, issued Oct. 7, 1997, to V. J. Hruby, M. E. Hadley and F. Al-Obeidi, entitled Cyclic Analogs of Alpha-MSH Fragments, and in U.S. Pat. No. 5,714,576, issued Feb. 3, 1998, to V. J. Hruby, M. E. Hadley and F. Al-Obeidi, entitled Linear Analogs of Alpha-MSH Fragments. Additional related peptides are disclosed in U.S. Pat. Nos. 5,576,290, 5,674,839, 5,714,576 and 6,051,555. These peptides are described as being useful for both the diagnosis and treatment of psychogenic sexual dysfunction in males and females. These peptides are related to the structure of melanocortins. Other peptides are disclosed in U.S. Pat. No. 6,284,735 and U.S. Published Patent Applications Nos. 2001/0056179 and 2002/0004512.
In use of Melanotan-II, significant erectile responses were observed, with 8 of 10 treated men developing clinically apparent erections, and with a mean duration of tip rigidity greater than 80% for 38 minutes with Melanotan-II compared to 3.0 minutes with a placebo (p=0.0045). The drug was administered by subcutaneous abdominal wall injection, at doses ranging from 0.025 to 0.157 mg/kg body weight. Transient side effects were observed, including nausea, stretching and yawning, and decreased appetite.
It has long been believed that erectile response to melanocortin receptor-specific compounds, and both male and female sexual response in general, was related to the central tetrapeptide sequence, His6-Phe7-Arg8-Trp9 (SEQ ID NO:1) of native α-MSH. In general, all melanocortin peptides share the same active core sequence, His-Phe-Arg-Trp (SEQ ID NO:1), including melanotropin neuropeptides and adrenocorticotropin. MC3-R (the melanocortin-3 receptor) has the highest expression in the arcuate nucleus of the hypothalamus, while MC4-R (the melanocortin-4 receptor) is more widely expressed in the thalamus, hypothalamus and hippocampus. A central nervous system mechanism for melanocortins in the induction of penile erection has been suggested by experiments demonstrating penile erection resulting from central intracerebroventricular administration of melanocortins in rats. While the mechanism of His-Phe-Arg-Trp (SEQ ID NO:1) induction of erectile response has never been fully elucidated, it has been generally accepted that it involves the central nervous system, and binding to MC3-R and/or MC4-R, and according to most researchers, MC4-R.
Non-peptides have been proposed which alter or regulate the activity of one or more melanocortin receptors. For example, International Patent Application No. PCT/US99/09216, entitled Isoquinoline Compound Melanocortin Receptor Ligands and Methods of Using Same, discloses two compounds that induce penile erections in rats. However, these compounds were administered by injection at doses of 1.8 mg/kg and 3.6 mg/kg, respectively, and at least one compound resulted in observable side effects, including yawning and stretching. Other melanocortin receptor-specific compounds with claimed application for treatment of sexual dysfunction are disclosed in International Patent Application No. PCT/US99/13252, entitled Spiropiperidine Derivatives as Melanocortin Receptor Agonists. International Patent Application Nos. PCT/US00/14930, PCT/US00/19408, WO 01/05401, WO/00/53148, WO 01/00224, WO 00/74679, WO 01/10842 and the like disclose other compounds that may be so utilized.
Both cyclic and linear α-MSH peptides have been studied for sexual dysfunction; however, the peptides heretofore evaluated have had an amide or —NH2 group at the C-terminus. See, for example, Wessells H. et al., J Urology, cited above; Haskell-Luevano C. et al., J Med Chem 40:2133–39 (1997); Schioth H. B. et al., Brit J Pharmacol 124:75–82 (1998); Schioth H. B. et al., Eur J Pharmacol 349:359–66 (1998); Hadley M. E. et al., Pigment Cell Res 9:213–34 (1996); Bednarek M. A. et al., Peptides 20:401–09 (1999); U.S. Pat. Nos. 6,054,556, 6,051,555 and 5,576,290; and, International Patent Applications PCT/US99/04111 and PCT/US98/03298. While significant research has been conducted in an effort to determine the optimal structure of α-MSH peptides, including a variety of structure-function, agonist-antagonist, molecular modeling and pharmacophore studies, such studies have relied upon peptides with an art conventional —NH2 group at the C-terminus. Further, it has long been believed that biologically active neuropeptides, including α-MSH peptides, are amidated, with an —NH2 group at the C-terminus, and that such amidation is required both for biological activity and stability. See, for example, Metabolism of Brain Peptides, Ed. G. O'Cuinn, CRC Press, New York, 1995, pp. 1–9 and 99–101.
There remains a need for clinically effective melanocortin receptor-specific agents for use in treatment of sexual dysfunction, including particularly compounds that are effective at low doses, optionally may be administered by minimally invasive means such as oral or nasal administration, which have rapid onset of pharmacological action, and are without significant undesirable side effects.